A living hinge is a thin, flexible hinge connecting two relatively rigid parts. It is usually made from the same material as the rigid parts. It may be used to join rigid parts of a container, allowing them to bend along the line of the hinge. Polypropylene (PP) has traditionally dominated the living-hinge dispensing closure market as it is easily processed and has good hinge durability characteristics. Additionally, it is widely available and historically had favorable economics relative to polyethylene when utilized to these ends. These attributes coupled with PP's overall balance of properties make it a frequent choice for living-hinge closure applications, as well as many moulding applications.
In recent years, however, the economics of PP, which have historically been favorable, are no longer as cost effective. Indeed, PP has had increasing costs attributable to industry and market dynamics. The increased price volatility and high costs have led to a reduction of PP capacity in North America. Due to the foregoing, polyethylene now has more favorable economics as compared to PP than it has previously held.
In addition, it has become more desirable to have a closure that is made from the same type of polymer as the bottle to enable recycling of the entire container. Further, polyethylene has not always been a suitable replacement of PP due to poor living hinge durability. That is, polyethylene has not necessarily proven to be mechanically strong enough to last a large number of flexing cycles.
Accordingly, it may be desirable to produce polyethylene compositions having improved processability and/or longer living hinge durability.